Interestingly, Bloomberg recently had a video segment on brexit and which financial institutions would win or lose. Unfortunately, since it was video I now cannot find the darned thing.
Anyways, it had several parts and in each part there was a representative of a group of institutions that talked about his/her opinion. The groups, as well as i can remember them, were "the city elders", the fintech startups, the money managers, the large international banks, and the insurance companies.
Pretty much all of them were strongly against brexit, except for the "city elders." The city elders were represented by an old man that was very pro brexit, explaining that he would welcome lighter regulation in order to offer more services.
So apparently there are people within the city that yearn for brexit because they want to break free of EU regulations and handle hot money like in the old days. I am not sure exactly who "the city elders" are, but apparently they exist and wish for brexit.
So it seems like there are different forces with different priorities within the city. And while most institutions that actually perform most of the economic activity from small startups to giant banks and insurance companies are against brexit, there lurk a bunch of scheming old men in a dark smoke filled room sipping port and licking their chops in happy anticipation of brexit (at least that is how i imagine them ).
But as to the question whether these "city elders" caused brexit itself, I am not sure. It seems that brexit was caused merely by a bunch of opportunist politicians trying to take advantage of the anti-immigrant backlash, and that they were searching for influence and votes rather than success at brexit.